Although the study showed a considerable reduction in cumulative mortality rates, the Canadian investigators found that the absolute risk reduction in deaths from breast cancer -- that is, the difference in mortality rates among patient groups -- was less than 1%.
This led them to question whether radiation therapy is necessary after lumpectomy for DCIS.
"It is doubtful whether a benefit of this size is large enough to warrant radiotherapy," wrote the team led by Vasily Giannakeas of the Women's College Research Institute in Toronto.
Women with DCIS are often treated with radiation after lumpectomy, but it has been unclear whether this protocol actually reduces their risk of dying from the disease. So Giannakeas and colleagues investigated the extent to which radiation therapy reduces risk of death from breast cancer. The study included 140,366 women who had DCIS between 1998 and 2014, and it compared the following treatment protocols:
- Lumpectomy plus radiation versus lumpectomy alone
- Lumpectomy versus mastectomy
- Lumpectomy plus radiation versus mastectomy alone
Of the total cohort, 25% of the women were treated with lumpectomy, 46.5% with lumpectomy and radiation, and 28.5% with mastectomy.
For all study participants, cumulative mortality from breast cancer 15 years after initial treatment was 2%. But among patients treated with lumpectomy, the 15-year cumulative mortality rate from the disease was 25% lower in those who also had radiation than in those who did not.
|Effect of radiation therapy on DCIS mortality rate
||Lumpectomy plus radiation
|15-year mortality rate
The breast cancer mortality rate 15 years after treatment was further reduced by the combination of lumpectomy and radiation in women younger than 50 (1.59%), black women (0.87%), and women with estrogen receptor-negative cancers (0.57%).
Yet, on average, 370 women would need to be treated with radiation to save one life, according to Giannakeas and colleagues. This count was lower for black women (115 treated to save one life) and women younger than 50 (63 treated to save one life).
When the researchers calculated adjusted hazard ratios for death from breast cancer using a subset of propensity-matched patient pairings (done to account for variations that might predict which treatment protocol a woman would receive), the lowest ratio was the lumpectomy-plus-radiation group versus the mastectomy group.
|Adjusted hazard ratios by protocol for death from breast cancer
|Lumpectomy plus radiation vs. mastectomy alone
|Lumpectomy plus radiation vs. lumpectomy alone
|Mastectomy vs. lumpectomy
The study findings support the thesis that reductions in mortality rate are due to radiation therapy rather than to any particular breast cancer pathologic features, the researchers noted. Yet the small absolute risk reduction prompts the question of whether radiation therapy is warranted in women with DCIS, according to the group.
"Although the clinical benefit is small, it is intriguing that radiotherapy has this effect, which appears to be attributable to systemic activity rather than local control," Giannakeas and colleagues wrote. "How exactly radiotherapy affects survival is an important question that should be explored in future studies."
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